The present invention relates to methods of operation of droplet deposition apparatus, particularly inkjet printheads, comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and means actuable by electrical signals to vary the volume of said chamber, volume variation sufficient to effect droplet ejection being effected in accordance with droplet ejection input data.
Apparatus of this kind is well known in the art. EP-A-0 364 136 shows a printhead formed with a number of ink channels bounded on both sides by piezoelectric side walls which deflect in the direction of an electric field applied by electrodes on the wall surfaces, thereby to reduce the volume of the ink channel and eject a droplet from an associated nozzle.
Unlike xe2x80x98thermalxe2x80x99 printheads in which each ink channel is provided with a heater that can be actuated so as to generate a bubble of vapour which pushes ink out of the channel via an associated nozzle, there is no need for xe2x80x98variable volume chamberxe2x80x99 printheads of the kind described above to heat the ink in the channel.
However, the present inventors have discovered that heating of ink in the chambers of a xe2x80x98variable volume chamberxe2x80x99 printhead can take place, particularly when it is operated at high frequency. FIG. 1 of the accompanying drawings is a plot of droplet ejection velocity U against the amplitude V of the electrical signal applied to the piezoelectric side walls of a channel in a printhead of the kind shown in the aforementioned EP-A-0 364 136. Plot A corresponds to a droplet ejection rate of one drop every droplet ejection period, with each droplet ejection period lasting 0.25 milliseconds, whilst plot B corresponds to a droplet ejection rate of one drop every 66 droplet ejection periods. It will be seen that for a given amplitude V of the electrical signal, a significantly faster droplet will be ejected by the printhead when operating at the higher ejection rate than at the lower ejection rate. Such a velocity increase is attributable to a decrease in viscous losses during the droplet ejection process due to a reduction in the viscosity of the ink. This in turn is the result of an increase in the temperature of the ink between the two operating conditions A and B caused by the heating of ink in the channel which, it is believed, is due to inefficiencies in the printhead.
It will be appreciated that droplet ejection velocity has to be taken into account when synchronising droplet ejection from the printhead with the movement of the substrate relative to the printhead and that any variation in velocity will manifest itself as droplet placement errors in the final print. For example, the drop placement tolerance is frequently specified as one quarter of a drop pitch. Thus for a print matrix density of 360 dots per inch, the drop placement tolerance will be xcex94X=18 xcexcm. The variation in droplet ejection velocity, xcex94U is related to the dot placement tolerance by the formula
xcex94U=Ud2.xcex94X/h.Uh
where h is the flight path length (typically 1.0 mm), Uh is the printhead velocity relative to the print substrate (typically 0.7 msxe2x88x921) and Ud is the mean droplet ejection velocity.
For mean droplet ejection velocities of 5, 10 and 15 msxe2x88x921, the maximum acceptable variation in droplet ejection velocity is 0.65, 2.6 and 5.8 msxe2x88x921 respectively. Thus there is a substantially greater allowable tolerance in the drop velocity when the mean droplet ejection velocity takes a value greater than 5 msxe2x88x921.
On the other hand, there is maximum droplet ejection velocity (xe2x80x98threshold velocityxe2x80x99), Uthr, which corresponds to the onset of capillary instability. In variable-volume (piezoelectric) printers, the inventors have found Uthr to be usually in the range 12-15 msxe2x88x921 when continuous high frequency droplet ejection is sustained, although higher droplet ejection velocities can be obtained during short bursts of drop formation.
It will also be appreciated that the rate at which a particular chamber in a printhead is actuated will depend on the incoming droplet ejection input data (which will be determined by the image to be printed and generally vary from high to low). Thus in a printhead having a chamber operating in accordance with FIG. 1 and at a given amplitudexe2x80x94for example 35xe2x80x94of electrical signal V, droplet ejection input data causing the chamber to eject droplets frequently (equivalent to plot A) will result in a droplet velocity of 15 m/s whilst subsequent input data may only cause the chamber to eject droplets at a lower rate (equivalent to plot B) and consequently at a much reduced velocity of 2 m/s. Such a large (750%) variation in ejection velocity will clearly lead to inaccuracies in the placement of the droplets and a reduction in the quality of the printed image. Such an error may occur for every chamber in a multi-chamber printhead. The degree of difference between these two conditions increases with ink viscosity and also with operating frequency, making the control of this effect particularly important in high speed printers.
It will also be evident from FIG. 1 that there is only a narrow range of magnitude V of actuation waveformxe2x80x94denoted Wxe2x80x94over which droplet ejection at both high and low rates can be guaranteed. This in turn severely inhibits the operational flexibility of the printhead.
According to one aspect of the present invention, these problems are solved at least in preferred embodiments by a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and actuator means actuable by electrical signals to vary the volume of said chamber; volume variation sufficient to effect droplet ejection being effected in accordance with droplet ejection input data; the method comprising the steps of controlling said electrical signals such that the temperature of the droplet fluid in said chamber remains substantially independent of variations in the droplet ejection input data.
Such a method can avoid velocity variations between enabled channels due to variations in ink viscosity which in turn are attributable to temperature variants caused by differential actuation rates. Differential actuation rates are of course a result of differences in the droplet ejection input data between enabled channels.
This aspect of the present invention also comprises the method of operation of droplet deposition apparatus comprising first and second chambers each supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom and having actuator means actuable by electrical signals to effect droplet ejection selectively from said chambers in accordance with droplet ejection input data; the method comprising operating said actuator means to effect droplet ejection from the first chamber but not from the second chamber, and selectively electrically heating the fluid in the second chamber to reduce the difference in temperature between fluid in the second chamber and fluid in the first chamber.
Again, by reducing variation in the temperature of the droplet fluid between first and second chamber, viscosity-related droplet ejection speed differences can be reduced.
Thus again according to the invention there is provided a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and actuator means actuable by electrical signals to effect droplet ejection from the chamber in accordance with droplet ejection input data; the method comprising controlling said electrical signals such that the maximum droplet ejection velocity lies just below a threshold velocity (Uthr), as hereinbefore defined and the variation in the droplet ejection velocity due to variations in the temperature of the droplet fluid in said chamber lies within a range determined by constraints in drop landing position.
According to another aspect of the present invention there is provided a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid, a nozzle communicating with the channel for ejection of droplets therefrom and actuator means having first and second electrodes and actuable by a potential difference applied across first and second electrodes to effect droplet ejection from the chamber via the nozzle; the method comprising the steps of applying to the first electrode a first non-zero voltage signal for a first duration, applying to the second electrode a second non-zero voltage signal for a second duration, the first and second voltage signals being applied simultaneously for a length of time less than at least one of said first and second durations.
This second aspect allows short potential pulses to be generated using voltage waveforms that are of longer duration and thus simpler to generate, not requiring complex and expensive circuitry. Such short pulses, whilst generally applicable in printhead operation, are of particular use when implementing the other aspects of the invention described above.
The novel principle of selectively electrically heating non-firing (drop ejecting) chambers in a droplet deposition apparatus to reduce temperature variations between the fluid in different chambers is applicable to any such apparatus regardless of the mechanism by which the chambers are fired.
Thus in another aspect the invention provides a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and actuator means actuable by electrical signals to effect droplet ejection in accordance with droplet ejection input data; the method comprising the steps of controlling said electrical signals such that the temperature of the droplet fluid in said chamber remains substantially independent of variations in the droplet ejection input data.
According to another aspect of the invention there is provided a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and actuator means actuable by electrical signals to vary the volume of said chamber, volume variation sufficient to effect droplet ejection being effected in accordance with droplet ejection input data; the method comprising applying electrical signals so as to actuate said actuator means without effecting droplet ejection from said nozzle, the electrical signals being controlled in dependence on a further signal representative of temperature.
Such a method in preferred embodiments may facilitate more sophisticated control of the temperature of the droplet deposition fluid.
The present invention also comprises signal processing means configured for carrying out the aforementioned methods and droplet deposition apparatus incorporating such signal processing means.
Preferred features and embodiments of the present invention are set out in the subordinate claims and the description that follows.